An air interface is the wireless communications link between two or more communicating devices, such as a radio access network device (e.g., a base station, a NodeB, an evolved NodeB, a transmit point) and an electronic device (ED) (e.g., a user equipment (UE), a mobile phone, a sensor, a camera). Typically, both communicating devices need to know specific parameters of the air interface in order to successfully transmit and receive a transmission.
In 3GPP NR Release 15 (R15), a physical downlink shared channel (PDSCH) repetition for PDSCH with only one layer is supported. PDSCH repetition is intended to improve PDSCH reliability. However, the manner in which the mechanism is agreed to operate only allows the repetitions to be transmitted from the same transmit receive point (TRP) or beam. This is at least in part because there is only one quasi-co-location (QCL) information between the demodulation reference signal (DMRS) port or port group of the PDSCH, which is used for channel estimation, and a reference signal (RS) that is associated with an RS antenna port. QCL indicates a relationship between two RSs, and a specified set of channel parameters, for use in relation to ED channel estimation based on one of the two RSs. Related parameters such as Doppler shift, Doppler spread, average delay, and delay spread, can be derived from RS. However, in coordinated multipoint (CoMP) scenarios, multiple channel state information (CSI) processes can be configured for the ED to receive multiple CSI-RS from multiple transmission points (TPs), and the demodulation and timing reference for PDSCH may change dynamically. In such scenarios, QCL signalling may be used, to let the ED know which CSI-RS to use to derive the timing reference. Therefore, when a QCL information is configured for the DMRS port or port group and the RS port, the DMRS port can use the Doppler shift that has already been obtained by the RS port.
QCL information is defined in a transmission control indication (TCI) state that is configured by transmission of a higher layer parameter. The QCL assumption between the DMRS port or port group of the PDSCH and the RS port can be used by the ED when the ED is performing channel estimation for a link between a TRP and an ED. Channel estimation is performed by using received DMRSs and determining how the DMRS changes due to channel effects. The ED can utilize the QCL information defined between the DMRS of the PDSCH, which is associated with a DMRS port, and the RS, which is associated with a RS antenna port, when performing channel estimation, and use channel parameters (e.g. any of Doppler shift, Doppler spread, average delay, delay spread, Spatial Rx parameter) in the defined QCL information that have already obtained from the RS. Then the channel estimate is used in demodulating PDSCH received in the port(s) that is the same as DMRS port(s) of PDSCH.
In R15, the number of PDSCH repetitions is configured by a higher layer parameter, namely pdsch-AggregationFactor, having possible values of 2, 4 or 8. TS38.214 states “when the UE is configured with aggregationFactorDL>1, the same symbol allocation is applied across the aggregationFactorDL consecutive slots. The UE may expect that the TB is repeated within each symbol allocation among each of the aggregationFactorDL consecutive slots and the PDSCH is limited to a single transmission layer.”
The R15 mechanism for supporting multiple PDSCH repetitions mentioned above has drawbacks. As agreed upon in R15, DCI is transmitted once for all PDSCH repetitions. Only one TCI state is defined in the DCI. The one TCI state has only one QCL information. Because a QCL information defines channel parameters associated with a channel for a particular transmission point or beam, if there is only one QCL information, all repetitions are considered to be transmitted from one TRP or one beam.
Accordingly, there is a desire for an improved method of supporting multiple PDSCH repetitions received from multiple TRPs or multiple beams.